This invention relates to a method of and an apparatus for treating the outer surface and inner surface of a pipe of a thermoplastic resinous material produced by extrusion so as to increase the luster on the surface of the pipe and additionally increase the stability of molding of the pipe. This invention also relates to a method of and an apparatus for smoothening the inner surface of a pipe of a thermoplastic resinous material which is produced by extrusion from the extruder.
Pipes of a thermoplastic resinous material are generally produced by extrusion molding. In this process, a pipe of a thermoplastic resinous material extruded in a molten condition is passed through a sizing die in a sizer to be sized. The luster on the surface of such pipe may vary depending on the temperature of the pipe at which it passes through the sizing die. In an ordinary thermoplastic resinous material, the temperature showing the highest value of its physical character is different from the temperature at which its surface luster becomes highest. Generally, it is more important to improve the physical character. Thus, it has hitherto been customary to use the temperature which shows the highest value of the physical character at the sacrifice of the surface luster.
Pipes of a thermoplastic resinous material lose their surface luster due to the influence of temperature. This defect of pipes of a thermoplastic resinous material is attributed to the intermittent movement of the pipes generally referred to as a stick and slip which is caused to occur by the adoption of a vacuum method or internal pressure method when the pipes are passed through the sizing die.
Pipes of a thermoplastic resinous material produced by extrusion molding by means of an extruder are used for producing various final products including, for example, bottles and other hollow articles which are made by blow molding from the thermoplastic resinous pipes. Methods of the prior art of producing transparent bottles by using a pipe of a thermoplastic resinous material, such for example as polypropylene, and elongating the same biaxially have the disadvantage of unable to produce transparent bottles because of the appearance of a cloud in the bottles produced. Research carried out to obviate this disadvantage has shown that the cloud in the body of the bottle is due to the roughness of the inner surface of the bottle, and the surface roughness exists on the inner surface of the pipe which is produced by extrusion as an intermediate product. More specifically, the refraction and irregular reflection of light by minuscule irregularities on the inner surface of the product are responsible for the cloud. It is possible to render smooth the outer surface of the intermediate or final product by mechanical means. However, in the case of hollow articles produced by blow molding, difficulty is experienced in mechanically smoothening the inner surface of the intermediate product or final product.